The biggest issue you will have with such a long beam is that it will
want to bend laterally when multiple people start swinging on it,
particularly when they are all swinging in the same direction. I
design playsets for my website, www.arciplay.com, and though I do not
have a swing beam that meets your exact requirements, you could
lengthen the playset 1.2 swing beam a few feet if you replace the beam
I have shown with an 18' long 6"x6" or better yet 8"x6" (the base
design shows a 4"x6") (a beam like this will be very expensive) the
cables in the design will support the beam laterally. I do not
recommend using 3 2x6" together as you mention unless they are pre-
engineered and fabricated as 3 already combined 2"x6"s.